1. Field of the Invention
The invention relates to a communication device which receives digital data via a radio mobile communication system and, in particular, to a digital demodulating circuit in the communication device.
2. Description of the Related Art
In a previous digital cellular phone used in a mobile radio communication system of the type described, radio signals are firstly received through an antenna and a desired frequency signal is then selected from the received signals to be subjected to demodulation and to obtain a demodulated signal. The demodulated signal is converted into a voice signal and is consequently supplied to a receiver of the cellular phone to be reproduced into a voice.
In a mobile radio communication system which includes a digital cellular phone as mentioned above, xcfx80/4 shift DQPSK is often adopted as a modulating method on a transmission side while a delay detecting method is used on a reception side to demodulate signals which are subjected to the modulating method.
A digital cellular phone of the type described often has a receiver which uses such a delay detecting method. Specifically, such a receiver comprises a phase detecting circuit, a phase difference detecting circuit, and a clock reproducing circuit. The phase detecting circuit detects a phase of the selected signal on the basis of reproduced symbol clock signals reproduced by the clock reproducing circuit and the selected signal, and produces a phase signal. The phase difference detecting circuit holds a phase signal at a time point of the symbol clock signal, receives the next phase signal at the next time point of the symbol clock signal, and produces a difference of the both phase signals as a phase shift signal. The clock reproducing circuit reproduces a symbol clock signal using the phase signals, and supplies the symbol clock signal to the phase detecting circuit and the phase difference detecting circuit.
Phases of the selected signals are shifted by xc2x1xcfx80/4 or xc2x13xcfx80/4 each other at each time point of the symbol clock signal and digital data signals are demodulated based on the values of the shifted phases.
A similar configuration of demodulating device is disclosed in Japanese Laid Open Publications Nos. H06-6398 (namely, 6398/1994), H06-261085 (namely, 261085/1994), H10-190568 (namely, 190568/1998), H09-18405 (namely, 18405/1997), H10-107730 (namely, 107730/1998), H10-257004 (namely, 257004/1998), H09-321694 (namely, 321694/1997), and H10-145467 (namely, 145467/1998).
At any rate, to demodulate a signal correctly, symbol timing of the selected signal should be coincident with the symbol clock signal from the clock reproducing circuit.
Herein, it is to be considered that such a digital cellular phone is repeatedly turned on or off by a user or a subscriber, which may be called intermittent receiving. In this case, power supplies to circuits are suspended in an inactive state. Under circumstances, when the power supplies are restarted, timing of the symbol clock signal would not be coincident with the symbol timing of the selected signal. In such a case, the selected signal can not be received without timing adjustment again. In other words, timing adjustment is indispensable to make the timing of the symbol clock signal and the symbol timing of the selected signal coincident with each other.
Thus, in a digital cellular phone which carries out the intermittent receiving, it is required to keep at least the clock reproducing circuit active even if the power supplies for main circuits are suspended. Further, it is also required to keep a demodulation clock signal (a clock signal used for demodulating) active since the symbol clock signal is generated based on the demodulation clock signal which has a frequency higher than that of the symbol clock signal. Furthermore, it is also required to keep an oscillator active which generates a reference frequency signal since the demodulation clock signal is generated by using the reference frequency signal.
In this way, keeping a plurality of circuits active causes power consumption to increase. In particular, when the demodulation clock signal is being generated and the oscillator is being generating a signal which has the reference frequency, electric power is seriously consumed during a waiting state as a frequency of a signal used in the circuits becomes high. For example, in the previous circuit, the reference frequency is 14.4 MHz and the demodulation clock signal is 2.688 MHz.
A portable data communication device, such as the cellular phone, requires portability from its characteristics. Therefore, it is important to decrease power consumption and extend a life of battery for such a device.
Therefore, it is an object of the invention to provide a cellular phone which is capable of decreasing power consumption and which consequently has a long life time without exchanging its battery.
Also, it is an object of the invention to provide a cellular phone which can reduce power consumption while the cellular phone is put into a waiting state.
Further, it is an object of the invention to provide a cellular phone which can quickly respond to a proper symbol clock signal as well as to reduce power consumption of the cellular phone during the waiting state.
Still further, it is an object of the invention to provide a symbol timing holding circuit which can be used for a digital cellular phone and can quickly and correctly generate a symbol clock signal when a call is restarted.
Still further, it is an object of the invention to provide a time measuring unit which can measure a correct time with decreasing power consumption by measuring a time for intermittent reception of a radio signal at a communication device such as a digital cellular phone, using a low frequency signal and a high frequency signal.
According to a first aspect of the invention, there is provided a communication device which intermittently takes receiving status to receive and demodulate a radio signal, and non-receiving status not to receive and demodulate the radio signal. The communication device comprises two oscillators each of which generates a frequency of signal (the frequencies being differ from each other), and a calculator which determines a time period of the non-receiving status with reference to a relationship between the frequencies of the signals from the two oscillators.
According to a second aspect of the invention, a communication device which receives digital data via a radio signal is provided. The communication device comprises a first oscillator which generates a first frequency of a first signal, a second oscillator which generates a second frequency of a second signal, a symbol clock circuit which generates a symbol clock signal by using the second signal, and a controller which suspends supply of the second signal to the symbol clock circuit to stop generating the symbol clock for a predetermined time period when the digital data is expected not to be received and which controls to suspend power supplies to the first oscillator and the second oscillator for a time period which is shorter than the predetermined time period and which is a part of the predetermined time period.
According to a third aspect of the invention, there is provided a method of measuring a time period of non-receiving status used in a communication device which intermittently takes receiving status to receive and demodulate a radio signal, and non-receiving status not to receive and demodulate the radio signal. The method comprises the steps of generating a frequency of signal and the other frequency of signal, and determining the time period of the non-receiving status by calculating with the frequencies of the signals.
According to a fourth aspect of the invention, there is provided a method of controlling power supplies of circuits in a communication device which receives digital data via a radio signal and comprises a first oscillator which generates a first frequency of a first signal, a second oscillator which generates a second frequency of a second signal, and a symbol clock circuit which generates a symbol clock signal by using the second signal. The method comprises the steps of suspending supply of the second signal to the symbol clock circuit for a predetermined time period to stop generating the symbol clock, and suspending power supplies to the first oscillator and the second oscillator for a time period which is shorter than the predetermined time period and which is a part of the predetermined time period.
According to a fifth aspect of the invention, there is provided a recording medium readable by a computer, tangibly embodying a program of instructions executable by the computer to perform a method of measuring a time period in a communication device which receives digital data via a radio signal. The method comprises the steps of determining a first repeat number of cycles of a first signal and a second repeat number of cycles of a second signal having a frequency higher than the frequency of the first signal so that the sum of the product of a time period of a cycle of the first signal by the first repeat number and the product of a time period of a cycle of the second signal by the second repeat number is equal to a predetermined time period or is approximated in a range of the predetermined time period, and measuring the predetermined time period by repeating a cycle of the first signal a plurality of times corresponding to the first repeat number and repeating a cycle of the second signal a plurality of times corresponding to the second repeat number.
According to a sixth aspect of the invention, there is provided a recording medium readable by a computer, tangibly embodying a program of instructions executable by the computer to perform a method of controlling power supplies of circuits in a communication device which receives digital data via a radio signal and comprises a first oscillator which generates a first frequency of a first signal, a second oscillator which generates a second frequency of a second signal, and a symbol clock circuit which generates a symbol clock signal by using the second signal. The method comprises the steps of suspending the second signal to the symbol clock circuit for a predetermined time period to stop generating the symbol clock signal, and suspending power supplies to the first oscillator and the second oscillator for a time period which is shorter than the predetermined time period and which is a part of the predetermined time period.